1. Field of the Invention
The present invention relates to a data recording and decoding system, and more particularly to a method and system for improving the reliability of digital recording by correcting peak shift data errors caused by the use of D.C. premagnetized (erased) magnetic medium. The present invention is particularly useful in connection with the writing of digital data on D.C. premagnetized (erased) magnetic medium, and with the accurate reproduction and decoding of such recorded data by magnetic read/write systems. More particularly it relates to a system for modifying the timing of a write current in order to avoid or eliminate peak shift due to magnetic recording on D.C. premagnetized magnetic medium. It relates mainly to high density recording systems, but improved performance will also be experienced in lower density recording systems.
2. Description of The Prior Art
Modern data processing systems include a variety of means for recording or writing digital data on a variety of recording medium. The present invention is described in the context generally of magnetic medium such as flexible magnetic tape; however, the present invention is applicable to any form of recording of digital data having predictable characteristics on any magnetic recording medium. The words "recording" and "writing" are used interchangeably herein to designate the recording of magnetic data signals on any form of magnetic recording storage medium.
It is desirable in such systems to maximize the reliability of data writing and reproduction, while at the same time maximizing the data throughput, all with a minimum of data errors. Such maximization is achieved in present day magnetic recording systems by increasing both the storage (writing) and reproduction (reading) speeds, and by increasing the data density (bits per unit area) on the magnetic medium. As the data recording density is increased, various undesirable effects are known to occur which cause data errors as a result of the interaction of the magnetic domains which comprise the adjacent data bits on the magnetic medium. Such interaction effects the density at which data can be reliably written and read. Various data encoding techniques have been developed for reducing these effects, including run length limited coding, group code recording, and others; however, in any encoding scheme, the above mentioned undesirable interaction effects occur at some given data density. One such undesirable effect is called "peak shift," and it most often occurs as a result of pulse crowding of the data bits on the magnetic recording medium. Peak shift is characterized by a shifting of the data transition locations from their proper (expected or timed) location. Peak shift will often result in a data error. This is due to the fact that in such recording systems individual data bits are recorded during a specific bit cell time in such a manner that a change of magnetic flux, or a magnetic flux peak at the discrete locations within the data bit cell or at its boundaries is read as being indicative of the recorded data. Such recorded data is written and then read on the magnetic medium as, for example, a logical "1" or a logical "0". Such flux transitions may be either a reversal of polarity or a change from one level of magnetization or flux to another. As used herein, a "flux reversal" is defined as that point which exhibits the maximum free space surface flux density normal to the surface of the magnetic medium, and is used interchangeably with the term "transition". In NRZ encoding, for example, such a transition occurs whenever a logical "1" is to be recorded. In MFM encoding, whenever a transition occurs at a boundary it is read as a logical "0," while a transition at the center of a data bit cell represents a logical "1". Also, as used herein, a "data bit cell" is defined as that time period during which one data representative flux transition should properly occur.
Most prior art peak shift problems have been due to and inherent in the coding scheme and the resulting transition between two or more sequentially occurring bit cells. For example, in high density recording, and in particular when no data transition (polarization reversal) is present for two or more sequential bit cells, the point in time on the magnetic medium at which the next following transition peak occurs is found to shift from its proper (expected) place. This causes the width of the bit cell to vary, with the result that normal decoding circuits may decode (read) erroneous data due to loss of synchronization of incoming data, or due to the decoding of a transition (polarization reversal) occuring in an improper (adjacent) bit cell. When the pulses are close together the trailing edge of a previous pulse, or the leading edge of a succeeding pulse may extend past the bit cell of the pulse peak under consideration. When this happens the time of occurrance at the peak will be shifted toward either the preceding or succeeding pulse depending on which pulse's edge is overlapping the peak. Descriptions and drawings of this peak shift phenomenon are set forth in U.S. Pat. No. 3,623,041 (MacDougall) and 3,537,084 (Behr).
Additionally, since the art has advanced to higher recording densities it has become a common practice to not magnetically saturate the magnetic medium as deeply as was the common practice in lower density recording systems. However, this lack of saturation of the medium has presented an undesirable effect during the writing of new data over old magnetically recorded data. This is due to the fact that recording at lower frequencies, but at relatively high magnetic saturations may penetrate the medium more deeply than subsequent overwriting at higher frequencies, but with less magnetic saturation. In order to avoid difficulties due to unerased data which might remain after such overwriting, it is now common practice to magnetically erase the magnetic recording medium before recording (overwriting) on it. In the prior art, both A.C. and D.C. magnetic erase techniques have been used for erasure. The A.C. method of erasure is commonly used, but is relatively more costly in terms of the erase head and the circuit for the erase head which are required. However, despite its higher cost, A.C. erasure has the advantage of not producing premagnetized magnetic media which may introduce additional errors into the subsequently recorded data. By comparison, D.C. premagnetization (erasure) of magnetic medium requires a less expensive erase head and circuit, but has a tendency to introduce yet another kind of peak shift error into the data recorded and then read from such D.C. premagnetized medium. These errors in D.C. premagnetized (erased) magnetic medium are seen as a peak shift which occurs when signals are subsequently recorded on the medium, which recorded signals traverse (are polarized) in the same direction as the polarization of the D.C. premagnetized magnetic media.
Various approaches have been taken in the prior art in an effort to avoid or compensate for peak shift in magnetic recording; however, such prior art approaches have been primarily directed to the correction of errors caused by sequencing, rather than to errors inherent in the character of the polarization of the magnetic recording medium, at the time it is written, for example due to D.C. erasure.
One class of solutions to peak shift problems caused by sequencing has entailed compensating the signal at the time the data is written or encoded, e.g. when it is known that a particular peak will be shifted in a particular direction, by writing or encoding the data earlier or later in an effort to compensate for the shift which is expected to occur. This solution was at first treated as unsatisfactory since the pulse adjacent the pulse being compensated will also cause peak shift in the opposite direction. Thus, for a time it was taught by the prior art that using techniques of writing earlier or later were of little value as a means to avoid peak shift, and that in fact such techniques would cause other problems. More recently, techniques for timing adjustment have been found which do not cause opposite peak shift, but they have been quite complex. In any event, no prior art technique is known for adjusting data encoding to compensate for peak shift error due to the use of D.C. premagnetized medium.
U.S. Pat. No. 3,503,059 (Ambrico) discloses the most commonly used method of correcting pulse shift errors due to sequencing. Ambrico teaches the use of minor distortions (step write compensation) in the magnetic flux after each major transition so that upon read-back the peaks will occur at the proper time. U.S. Pat. No. 3,573,770 (Norris) employs the same technique, but different means to avoid peak shifting. U.S. Pat. No. 3,623,041 (MacDougall) uses a different approach which is quite successful as well. MacDougall provides a new system of encoding which has fewer signal transitions. Fewer transitions means fewer pulses, and therefore, less pulse crowding for similar data rates or intensities. U.S. Pat. No. 3,537,084 (Behr) employs a technique in which writing is not modified, but in which the read back is compensated.
Another prior art technique is described by U.S. Pat. No. 3,879,342 (Patel) in which a means of compensation for peak shift present in three frequency coding is introduced. A pulse shift circuit advances or delays the writing pulses; however, it is a complicated system in which three separate clock signals are required. In U.S. Pat. No. 3,483,539 (Poumakis) high density self-clocking information storage along a magnetic track is taught in which it is necessary to determine whether each successive pulse occurs after a predetermined short interval following the preceding pulse, or after a predetermined long interval following the preceding pulse. Poumakis then distinguishes between long and short intervals and repositions each pulse which occurs less than a minimum short interval following a preceding pulse so that each pulse always occurs after such a minimum interval.
In U.S. Pat. No. 4,000,513 (Precourt) peak shift due to pulse crowding of data recorded on a magnetic medium is reduced by preemphasizing the recorded data time pattern in order to compensate for peak shift of the magnetic pattern recorded on the magnetic medium. Preemphasis of recorded peak shift errors is accomplished either by delaying or advancing the time when a particular peak shift data transition caused by the encoding data pattern will occur. This complex system causes the data to either be delayed or advanced before it is written, thereby compensating for peak shift error which would otherwise be present in the recorded data.
Again it is noted that no known peak shift compensation system has been divised which simply and inexpensively adjusts the timing of the encoding signal used with D.C. premagnetized medium.